1. Field of the Invention
The present invention relates to the acquisition and evaluation of mass spectra in Fourier transform (FT) mass spectrometers in which ions oscillate on trajectories at mass specific frequencies and the ion motion is detected as a time-domain signal.
2. Description of the Related Art
Today, the two main classes of Fourier transform mass spectrometers are ion cyclotron resonance (ICR) mass spectrometers and electrostatic Kingdon ion traps with a harmonic potential along a longitudinal direction. In general, FT mass spectrometers comprise a measuring cell in which analyte ions oscillate along one or two spatial dimensions at frequencies being specific to their mass-to-charge ratio. The motion of the oscillating ions is recorded as a time-domain signal, e.g., by measuring the image current induced on detection electrodes of the measuring cell. A mass spectrum or, more generally, separated mass signals are obtained by applying a spectral decomposition, e.g., by a Fourier transform, or a parameter estimation method, e.g., a filter diagonalization method (FDM), to the time-domain signal. The amplitude and frequency of a mass signal relate to the mass-to-charge ratio and abundance of an analyte ion species. A calibration is needed to assign the frequency of a mass signal to a mass-to-charge ratio.
ICR mass spectrometers are based on the cyclotron frequency of ions in a magnetic field. Analyte ions are commonly introduced into an ICR cell and then excited to orbital motion around a longitudinal axis. The orbiting ions induce image currents on detection electrodes of the ICR cell. The image currents are recorded as a time-domain signal (“transient”) and converted into a mass spectrum, most often by a Fourier transform. The frequency axis of the mass spectrum can be converted into a mass axis since the cyclotron frequency is inversely proportional to the mass to charge ratio. The analyte ions are trapped radially by the magnetic field and longitudinally by electric potentials along the longitudinal axis of the measuring cell.
FIG. 1A shows a cylindrical ICR cell according to the prior art. The ICR measuring cell comprises two trapping end cap electrodes (11) and (12) which have the form of plane apertured diaphragms. The analyte ions are introduced into the ICR cell through the apertures. Four longitudinal sheath electrodes (13) are arranged between the trapping electrodes (11) and (12) which have the form of parallel sections of the cylindrical surface. Of the four longitudinal electrodes (13), two opposing electrodes serve to excite the ions to cyclotron orbits and the other two serve as detection electrodes to measure the image currents.
FIG. 1B shows a cylindrical ICR cell as disclosed in U.S. Pat. No. 8,704,173 by Nikolaev et al. (Title: “Ion cyclotron resonance measuring cells with harmonic trapping potential”). The twenty-four sheath electrodes (21) to (44) of the cylindrical measuring cell are divided by separating gaps with parabolic shape into eight digon-shaped ((21) to (28)) and sixteen curved triangular sheath electrodes, (29) to (44). Only electrodes (21) to (23) and (29) to (36) are visible in the figure. The ICR cell is closed at both ends by end cap electrodes (20a, 20b) which have a rotationally hyperbolic form. The aperture in end cap electrode (20a) allows for the introduction of analyte ions on the central axis along the magnetic field lines. A single trapping voltage is applied to the triangular sheath electrodes (29) to (44), and the endcaps (20a, 20b), generate an axial trapping potential distribution in the interior of the cell. The potential has a parabolic profile in an axial direction for orbiting ions. The digon-electrodes (21) to (28) are either used as excitation electrodes or detection electrodes.
The class of electrostatic Kingdon ion traps with a harmonic potential along a longitudinal direction comprises two different types of traps: orbital-Kingdon traps and the oscillational-Kingdon traps.
Orbital-Kingdon traps are described in U.S. Pat. No. 5,886,346 (Makarov: “Mass spectrometer”), and consist of an outer barrel-like electrode and a coaxial inner spindle-like electrode. Analyte ions orbit around the inner electrode (to which an attracting potential is applied) while they oscillate at the same time along the axis of the inner electrode (longitudinal direction) in a parabolic electric potential.
Oscillational-Kingdon traps are described in U.S. Pat. No. 7,994,473 (Koster: “Mass spectrometer with an electrostatic ion trap”). An oscillational-Kingdon trap can, for example, comprise an outer electrode and two spindle-shaped inner electrodes with ion-attracting potentials applied to each inner electrode. The outer electrode and the inner electrodes are shaped and arranged such that a parabolic electric potential is formed along the axis of the inner electrodes. Analyte ions oscillate transversely in a plane between the two inner electrodes while they oscillate at the same time in the parabolic electric potential.
There is a third class of FT mass spectrometers using RF quadrupole ion traps with detection electrodes for measuring image currents induced by analyte ions which oscillate in the RF ion traps after introduction and excitation. A three-dimensional FT-RF quadrupole ion trap is disclosed in U.S. Pat. No. 5,625,186 (Frankevich et al.: “Non-destructive ion trap mass spectrometer and method”). A linear FT-RF quadrupole ion trap in which analyte ions oscillate between two pole rods is disclosed in U.S. Pat. No. 6,403,955 (Senko: “Linear quadrupole mass spectrometer”).
U.S. Pat. No. 5,679,950 (Baba: “Ion trapping mass spectrometry method and apparatus therefor”) discloses three-dimensional and linear RF quadrupole ion traps comprising a laser device for generating a cooling laser beam and a photo detector. Analyte ions generated in the ion trap are supplemented by a specific ion species which is trapped concurrently in the RF ion trap. The added ions generate fluorescence of high intensity and are called probe ions. A light beam is introduced into the RF ion trap to excite the probe ions optically whereby the motion of the probe ions is observed. A supplemental AC electric field is applied to the RF ion trap while being scanned in terms of its frequency. When the secular frequency of the analyte ions coincides with the frequency of the AC electric field, the analyte ions oscillate by resonance. The oscillating analyte ions disturb the motion of the probe ions due to Coulomb collision with the probe ions. Changes in the motion of the fluorescent probe ions are detected optically providing a means of determining how the analyte ions oscillate by resonance. Baba refers to this analyzing scheme as fluorescent mass spectrometry.
U.S. Pat. No. 7,964,842 (Köster: “Evaluation of frequency mass spectra”) describes methods for evaluating mass spectra acquired with FT mass spectrometers. The methods are directed to detecting and correcting a parameter drift that occurs during recording of a time-domain signal. The detection of the drift can comprise an analysis of a frequency component, i.e., the time-domain signal generated by a single ion species, to determine whether the instantaneous frequency of the frequency component is constant during recording of the time-domain signal. The instantaneous frequency as a function of time can be determined by applying a short-time Fourier transform to the time-domain signal or from other time-frequency representations of the time-domain signal.